The present invention generally relates to devices for installing fasteners, and more specifically relates to a high performance jaw system for installing fasteners, such as but not limited to blind fasteners.
FIG. 1 provides a cross-sectional view, and FIG. 2 provides an exploded perspective view, of a pulling head 10 which is presently commercially available. The pulling head 10 is configured for engagement with a conventional riveter, such as the G746A power riveter, the G747 power riveter, the G704B riveter, the G30 hand riveter or the G750A hand riveter, each of which is commercially available from Cherry Aerospace.
The pulling head 10 includes a sleeve 12 which is generally cylindrical and has a threaded bore 14 at one of its ends 16 for receiving a nosepiece 18 in a threaded engagement. A collet 20 is disposed in the sleeve 12, and the collet 20 is also generally cylindrical. The collet 20 includes an internally threaded portion 22 which is configured to engage a piston of the riveter in a threaded engagement. Inside the collet 20 sits a set of two or three jaws 24, each of which can be cast from a low grade steel, which is surface hardened or machined from tool steel, and includes teeth 26 which generally match annular serrations of a break stem of the fastener to be installed. The jaws 24 are kept generally together via an o-ring 28 which engages a notch 30 provided on an outside surface 32 of each of the jaws 24.
The front end 34 of each of the jaws 24 is tapered and configured to contact a corresponding angled surface 36 on the nosepiece 18 when the jaws 24 are forward in the collet 20. An external surface 38 of each of the jaws 24 is angled and configured to engage a corresponding angled surface 40 on the inside of the collet 20. In back of the jaws 24 is a jaw follower 42. Specifically, the back end 44 of each of the jaws 24 provides an angled surface 46 which is configured to engage a corresponding angled surface 48 on the jaw follower 42. The jaw follower 42 is generally cylindrical and engages an end 50 of a compression spring 52. An opposite end 54 of the compression spring 52 engages a shoulder 56 which is provided proximate an end 58 of a sleeve 60. In addition to the angled surface 48 on the jaw follower 42, the jaw follower 42 includes a bore 62. The sleeve 60 also includes a longitudinal bore 64.
The pulling head 10 shown in FIGS. 1 and 2 is configured such that the jaws 24 rest on the jaw follower 42 and are kept in position with the assistance of the compression spring 52. The jaws 24 are relatively small and stubby and are unable to function unless they are pushed forward with significant force by the jaw follower 42 (viz-a-viz the compression spring 52).
The pulling head 10 shown in FIGS. 1 and 2 works relatively well for fasteners requiring lower installation loads, with break stems having serrations that have a relatively fine pitch, when the load per tooth and the installation shock is low. However, for higher loads and installation shocks (such as is required for installing steel blind bolts), the pulling head 10 shown in FIGS. 1 and 2 has a low life and is not very reliable.
Some of the factors contributing to the pulling head 10 shown in FIGS. 1 and 2 having a low tool life include: high load per jaw tooth due to so few teeth 26 being in engagement with the break stem of the fastener; low life of the spring 52 used to keep the jaws 24 in position and to close them during operation, causing the jaws to mis-align or tumble; the loose jaws are difficult to assemble, and they are prone to mis-aligning and tumbling during operation, the jaws are far away from the stem to be grabbed, creating insurmountable jaw engagement issues that cause installation failures and frequent jaw breakages.
As mentioned above, during operation the compression spring 52 behind the jaw follower 42 takes a set due to very high shock loads and axial forces weakening the push on the jaws 24. While a weaker spring causes instability of the jaws causing them to possibly tumble and break, increasing the spring force tends to cause the jaw follower to fail.
The jaw life expectation of the pulling head 10 shown in FIGS. 1 and 2 is typically a few hundred installations. In a high volume production environment, this is unacceptable. The jaws tend to fracture on the conical area, and it has been found that changing to tougher or stronger materials seems to have little impact on this type of failure.
U.S. Pat. No. 4,347,728 discloses a jaw system which provides three small jaws which are vulcanized on a rubber tube. The section of the jaws is relatively large, in order to provide stiffness. Although the pulling head design disclosed in the '728 patent partially solves the jaw alignment problem and makes the assembly operation easier, certain issues remain unresolved making the design an incremental improvement at best. For example, the alignment of the serrations of the jaws with those of the break stem still remain an issue. The three jaws are cast, and the overall length cannot be accurately controlled. Therefore, slight variations in the jaw length will position the teeth of the jaws off from each other, causing uneven loading of the jaws. Also, since the jaw length and number of serrations in engagement with the break stem is too short for the extremely high loads and shocks, jaw life is still relatively low (and not significantly different from that of the pulling head shown in FIGS. 1 and 2). Furthermore, due to the design being very size specific, the jaw radial expansion is constrained. Additionally, the rubber tube is easily damaged by the stems as they eject, due to their moving at high velocity. Moreover, because of the configuration of the jaws, they have to be relatively far from the active area of the installation, so grip capability, stem length, and jaw engagement are typically insurmountable problems. Finally, the bulkiness of the jaws and the length of the rubber tube lead to larger tools, and in the aerospace industry tool compactness is critical.